1. Field of the Invention
The present invention relates to communications receivers and more particularly to techniques for generating precise quadrature reference signals for use in the same.
2. State of the Art
Direct conversion receivers are known in the art as exemplified by U.S. Pat. No. 6,061,551, incorporated herein by reference. Such receivers have various advantages over conventional superheterodyne receivers. Regardless of the receiver architecture, however, there is typically a need to generate quadrature reference signals, e.g., a pair of local oscillator (LO) signals phased-shifted by 90°. Both analog and digital techniques have been employed for this purpose. In the case of an analog phase-shift network, because the phase-shift network is narrowband, inaccuracies result at frequencies separated from the nominal design frequency. As data rates and constellation complexity increase, these inaccuracies become a significant impairment. In the case of digital techniques, an input signal is required that is a frequency multiple of the desired LO frequency. This input signal is frequency divided, typically multiple times. Because of the high switching speeds involved, such circuitry tends to be fairly power hungry.
U.S. Pat. No. 4,475,088, incorporated herein by reference, describes an alternative architecture for achieving quadrature alignment, i.e., for generating a pair of quadrature signals having a precise 90.degree. phase offset. As illustrated in FIG. 1, a radio frequency (RF) input signal to be detected is coupled to a first input of first and second quadrature detectors, 10 and 12. A local oscillator 14 provides an RF signal which is divided into two reference signals separated from each other by a phase difference of approximately 90.degree. by a variable phase-shift network 16. These signals are coupled to the second inputs of the quadrature detectors 10 and 12. The output signals of the quadrature detectors 10 and 12 are the conventional I and Q signals associated with quadrature detection systems. The I and Q signals appearing at the outputs of the mixers 10 and 12 are coupled to a phase error detection network 18 implemented, for example, by a digital computer suitably programmed or with analog circuitry. As a result of the calculations performed by the phase error detection network 18, a phase error signal is generated which is used to adjust the variable phase shift network 16 to adjust the phase of the reference signals of the quadrature detectors to reduce the phase error. If the I channel output signal is represented as (A sin x) and the Q channel output signal is represented as (B sin y), then the following equation for the phase error results:
      ϕ    ⁡          (      error      )        =      [                                        A            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            x                    +                      B            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            y                          A            -              0.707        ⁢                              B            -            A                    A                    -      1.414        ]  The phase error detection network 18 is based on the foregoing equation. No embodiment of the variable phase shift network is described.
There remains a need for a quadrature alignment technique that is simple in implementation and that achieves precise quadrature alignment.